This invention relates to a signal transmitting apparatus and, more particularly, to a signal transmitting apparatus for controlling the power-supply voltage of a transmission-line driving circuit, which transmits a signal to a transmission line, based upon the output signal amplitude of the transmission-line driving circuit.
A signal transmitting apparatus according to the prior art fixes the power-supply voltage, which is supplied to a transmission-line driving circuit (driver), at a certain value in conformity with the maximum value of the output signal amplitude. This method makes it possible to transmit a signal with little distortion. On the other hand, the average value of signal amplitude with respect to the value of the supplied power-supply voltage is small. As a consequence, the power of the transmitted signal with respect to the power consumed by the driver circuit is small and a problem that arises is poor power efficiency of signal transmission. In particular, with a multicarrier modulation method such as DMT (Discrete Multitone), described below, the ratio PAR (Peak Average Ratio) of momentary maximum output voltage of a signal to the average output voltage thereof is extremely high and the driver circuit is supplied with a high power-supply voltage owing to the momentary maximum value, which appears only rarely. Power efficiency, therefore, is low.
The ADSL (Asymmetric Digital Subscriber Line) scheme is a typical transmission scheme that uses DMT. In recent years, G.992.1 (G.dmt) and G.992.2 (G.lite) have been adopted as ADSL standards by the ITU. This will be described below taking G992.1 (G.dmt) as an example.
With DMT modulation, as shown in FIG. 8, a frequency band of 1.104 MHz is divided into M (M=256) multicarriers #1xcx9c#256 at intervals of xcex94f (=4.3125 KHz). The S/N ratios that prevail when a transmission is made in accordance with 4-QAM (Quadrature Amplitude Modulation) by each of the carriers #1xcx9c#256 are measured in advance and it is decided, depending upon the S/N ratios, with which modulation method among 4-QAM, 16-QAM, 64-QAM, 128-QAM . . . modulation methods data is to be transmitted in each carrier. For example, 4-QAM is assigned to a carrier having a small S/N ratio and 16-QAM, 64-QAM, 128-QAM . . . are assigned successively as the S/N ratio increases. It should be noted that 4-QAM is a modulation scheme in which two bits are transmitted at a time, 16-QAM a modulation scheme in which four bits are transmitted at a time, 64-QAM a modulation scheme in which six bits are transmitted at a time, and 128-QAM a modulation scheme in which seven bits are transmitted at a time.
FIG. 9 is a diagram useful in describing 16-QAM. A serial/parallel converter (S/P converter) 1 stores transmit data, which enters as a bit serial, in a buffer successively four bits at a time and outputs four bits as 2-bit parallel data (ai,bi), (ai+1,bi+1). A first binary/quaternary converter 2 converts the parallel data (ai,bi) to four values (xe2x88x923, xe2x88x921, +1, +3), and a second binary/quaternary converter 3 converts the parallel data (ai+1,bi+1) to four values (xe2x88x923, xe2x88x921, +1, +3). A carrier generator 4 generates a cosine wave cos (xcfx89ct) of frequency fc (xcfx89c=2xcfx80fc), and a phase shifter 5 shifts the phase of the cosine wave by 90xc2x0 to output a sine wave sin (xcfx89ct). An AM modulator 6 multiplies the output of the first binary/quaternary converter 2 by the sine wave sin (xcfx89ct), and an AM modulator 7 multiplies the output of the second binary/quaternary converter 3 by the cosine wave cos (xcfx89ct). An adder 8 combines the outputs of the AM modulators 6 and 7 and outputs the combined signal. By executing the operation described above, the 16-QAM modulator outputs signals having the illustrated two-dimensional signal point placement (constellation) in accordance with the combination of parallel data (ai,bi), (ai+1,bi+1). For example, if data divided into four bits at a time is 1001, 0011, 1100, 0110, the 16-QAM modulator outputs signals {circle around (1)}xe2x86x92{circle around (2)}xe2x86x92{circle around (3)}xe2x86x92{circle around (4)} in the constellation.
FIG. 10 is diagram useful in describing the principle of DMT modulation. From bit-serial transmit data, an S/P converter 11 stores a bit sequence that is to be transmitted within a certain period in an internal buffer and subsequently outputs the bit sequence to a carrier mapper 12. Data transmitted within this fixed period is referred to as a symbol. Since the QAM modulation scheme of each carrier is known, the carrier mapper 12 divides the one symbol""s worth of bit sequence bk-number of bits at a time in accordance with the QAM modulation scheme of each carrier and inputs the resultant bit sequence to a QAM modulator 13i of the particular carrier. As a result, the total number of output bits per symbol is xcexa3bk (k=1 to M). In this case, the carrier mapper 12 performs the bit division of one symbol successively in accordance with the QAM modulation scheme of the carrier, starting from carriers having a low frequency. A frequency multiplexer 14 frequency multiplexes the QAM signals output from the QAM modulators 13i of the respective carriers and outputs the multiplexed signal to a transmission line via a transmission-line driver circuit (not shown).
With the method described above, the number of QAM modulators required is equal to the number of carriers. Let Xk1, Xk2 represent the first and second halves, respectively, of 2xc2x7mk bits input to a kth QAM modulator (k=1, 2, . . . , M). If the following holds:
Xk=Xk1+jXk2
then the output signal of the frequency multiplexer will be a real-number portion of an inverse Fourier transform of Xk. Accordingly, transmission based upon DMT modulation is carried out by providing an arithmetic unit, which implements an IFFT (Inverse Fast Fourier Transform), instead of QAM modulators the number of which is the number (M) of carriers. FIG. 11 is a basic structural diagram of a DMT transmission circuit having an IFFT arithmetic unit. Components identical with those of FIG. 10 are designated by like reference characters.
When data of a complex frequency region enters via the S/P converter 11 and carrier mapper 12, an IFFT arithmetic unit 21 converts the frequency signal (frequency-region signal) Xk (k=1, 2, . . . M) of each carrier to a time-region signal x(m) by an IFFT arithmetic operation. If the time-region signal x(m) is illustrated upon enlarging the time axis, the result will be as shown in FIG. 12, by way of example, where m represents time at discrete time intervals xcex94t and m per symbol is equal to 1 to M.
A parallel/serial converter (P/S converter) 22 holds M-number of items of time-region data x(1) to x(M), which are output from the IFFT arithmetic unit 21, in an internal buffer and outputs this data in the order x(1), x(2) . . . x(M). A DA converter 23 converts the time-region data x(1), x(2) . . . x(M) to an analog signal and outputs the analog signal as well as a signal obtained by reversing the polarity of this analog signal. A band-pass filter (BPF) 24 passes only the necessary band components contained in the signals output from the DA converter and inputs these components to a transmission-line driver circuit (driver) 25. The driver 25, which has the structure of a differential amplifier, differentially amplifies the input signals and outputs the results to a transmission line 28 via resistors 261, 262 and a transformer 27.
In the case of an FDM (Frequency Divided Multiplex) scheme in accordance with G992.1 (G.dmt), all 256 carriers are allocated for (1) the upstream direction from the subscriber to the office and (2) the downstream direction from the office to the subscriber; the number of carriers for the latter is 224. Further, symbol frequency fc (=1/Tc) is 4.3125 kHz, the upper limit of average signal power value of transmission over each carrier is xe2x88x9240 dBm/Hz, and the transmission-line transfer impedance is 100xcexa9. Consider voltage amplitude Vcar and power P of a transmit signal that has been DMT-modulated based upon these signals.
First, from
Vcar=(Pxc3x97Z)1/2,
signal amplitude Var for one carrier is
Vcar={10(xe2x88x9240/10)xc3x9710xe2x88x923xc3x974.3125xc3x97103xc3x97100}1/2=208 [mV]
Accordingly, average amplitude Vavr of the DMT signal is
Vavr=(Mxc3x97Vcar2)1/2=3.11[V]
In order to obtain the necessary BER (Bit Error Rate) 10xe2x88x927, the PAR (Peak Average Ratio) must be made less than 18 dB. Accordingly, the following equation
xe2x80x8320 log(Vmax/Vavr)=18
holds and the maximum output amplitude Vmax is as follows:                               V          max                =                              V            avr                    xc3x97                      10                          18              /              20                                                              =                              3.11            xc3x97                          10                              18                /                20                                              =                      24.7            ⁢                          xe2x80x83                        [            V            ]                              
If it is attempted to send this maximum output amplitude signal to a line as is, a power-supply voltage of xc2x1Vdd=xc2x125 V, i.e., a voltage of 50 V or greater, will be applied to the driver 25. In consideration of the voltage withstand performance of the driver 25, actually the driver transmission voltage often is held to one-half of this using the transformer 27, the turns ratio of which is 1:2. However, even if the output amplitude of the driver 25 is reduced by the turns ratio of the transformer 27, it is still necessary that PAR=18 dB be satisfied. With DMT modulation, the probability that the maximum amplitude Vmax will occur is very low and the transition thereof is almost in the vicinity of the average value. As a consequence, the power efficiency of the transmission is very poor.
Power will now be considered specifically.
If the turns ratio of the transformer is 1:2, then the maximum output amplitude (on one side) of the driver 25 is
24.7/2=12.4[V]
and therefore a power supply of xc2x1Vdd=xc2x115 V is used. In this case, the driver transmission resistance value (primary resistance value) R is
xe2x80x83R=100/22=25 xcexa9
Since this resistance value is equal to the equivalent impedance of the line, the total impedance driven by the driver 25 is 25xc3x972=50 xcexa9.
Accordingly, the power necessary for the driver 25 to output the average amplitude signal Aavr is
P=(Vavr/R)xc3x97V=(3.11/50)xc3x9730=1.86 [W]
On the other hand, the power of the average amplitude signal is
Pxe2x80x2=(Vavr2/R)=3.112/50=193 [mW]
If P and Pxe2x80x2 are compared, it is obvious that almost all of the power is consumed by the driver 25 and that the portion used as the transmit signal is small, on the order of {fraction (1/10)}.
Thus, if a DMT-modulated signal is transmitted at a high, distortion-free quality by the above-described system, the power efficiency declines. In other words, with the prior art, a power-supply voltage that corresponds to the maximum amplitude of the transmit signal is always supplied to the driver even while the amplitude of the transmit signal is small. The result is poor power efficiency.
Accordingly, an object of the present invention is to make it possible to conserve power by raising the power efficiency of the transmission-line driver circuit (driver).
Another object of the present invention is to raise the power efficiency of the transmission-line driver circuit in a case where a multicarrier signal is transmitted, wherein the multicarrier signal is obtained by modulating carriers of multiple bands, the frequencies of which have been divided, by transmit data and combining the results.
When a transmission-line driver circuit, which transmits a signal to a transmission line, is supplied with a power-supply voltage from a power-supply circuit, the power-supply voltage value is controlled based upon the output signal amplitude of the transmission-line driver circuit. More specifically, the maximum value of a signal input to the transmission-line driver circuit in time units delimited at fixed time periods is detected, the target value of power-supply voltage supplied to the transmission-line driver circuit is decided based upon the maximum value, and a power-supply circuit is controlled in such a manner that the target value of power-supply voltage and actual value of power-supply voltage will agree. If this arrangement is adopted, the power-supply voltage can be controlled, at predetermined times, based upon the maximum amplitude of the signal input to the transmission-line driver circuit, i.e., the maximum amplitude of the signal output from the transmission-line driver circuit. As a result, it is possible to conserve power by raising the power efficiency of the transmission-line driver circuit.
In this case, the time unit""s worth of the signal used to decide the target value of the power-supply voltage is accumulated and the accumulated signal is input to the transmission-line driver circuit in parallel with control of the power-supply voltage in such a manner that the target value of the power-supply voltage and the actual value of power-supply voltage will agree. If this arrangement is adopted, accurate power-supply voltage control based upon the maximum value of the signal output from the transmission-line driver circuit can be carried out.
Further, the target value of the power-supply voltage is decided so as to vary at a rate lower than that at which the maximum value in each time unit varies.